In the graphic arts industry, which ranges from the publication of newspapers, books, and magazines to computer output printers, the printing process commonly used is an all or nothing binary process. This all or nothing process comprises depositing a dot of ink on paper wherever it is desired to print and to deposit no ink where the absence of an image is desired. This all or nothing process is acceptable and in fact desirable when alphanumeric characters or other symbols are printed. A problem arises, however, when images employing gray scales or light gradations in continuous tones are to be printed, such as in the printing of photographs. This problem is commonly solved by transforming the continuous tone of the original image into halftone or pseudo halftone images. Halftone images typically comprise a large number of ink dots of various sizes. The size of the ink dots correspond to the shades or tones to be reproduced. When the dots in the corresponding spaces on the paper between the dots are small compared to the visual acuity of the human eye, they are subliminal to the eye and not recognized. The dots and the spaces on the paper thus fuse visually and trick the eye into believing that various shades of continuous tones are seen. Pseudo halftone images refers to the process of reproducing the continuous tone images or gray scale with a printing device having a fixed printed spot size and fixed spot-to-spot spacing. The level of gray to be reproduced thus is represented by a number of dots which are printed out of a specified line segment or matrix array of printing positions. If the individual spots are sufficiently small, they effect a merger insofar as the eye is concerned to form a visual merger with the spaces between the dots to cause the eye to believe it is seeing various shades of continuous tones.
Eletonics have been applied to image printing at various times with respect to the creation of half tone or pseudo half tone images. One example is Richard G. Sweet et al U.S. Pat. No. 3,373,437, entitled "Fluid Droplet Recorder with a Plurality of Jets." This patent illustrates an ink jet printing mechanism wherein jets of ink are each caused to break into a uniform stream of droplets which are directed to impact the paper which is to be printed. Selected droplets which are not to be printed are charged as they break from the stream and are deflected by a constant electrostatic field to impact a gutter. A video intelligence signal representing the relative brightness or inverse gray scale of the image to be reproduced is sampled and applied to the charging electrode into a R-C time constant circuit. The sampled pulse thus decays at a rate determined by the R-C time constant. So long as the charging electrode stays above a predetermined threshold value, the droplets emanating from the jet are charged and deflected to the gutter, leaving the paper unmarked. Upon the signal dropping below the threshold value, the remaining drops of the stream are uncharged and impact the paper. The ratio of drops impacting the paper with respect to the total number of drops produced prior to application of the next video signal comprises the gray scale level of the sampled video signal. In a printing line made up of a string of ink droplets or their unprinted positions, the droplets would thus be grouped periodically in accordance with the periods of the video signal, the droplets appearing towards the end of each period and the spaces at the beginning of each period.
Another example is shown in IBM Technical Disclosure Bulletin, "Pseudo Half Tone for Representing Continuous Tone Images in Black-White Facsimile Systems," G. K. Machol, Vol. 9, No. 6, Nov. 1966, pages 636-637. This example illustrates the derivation of pseudo half tone matrices in accordance with a predetermined logic. The analog video signal is applied to a quantizer which samples the video signal and provides a binary coded output indicating the gray scale level of the sample point. The logic circuit then transforms the binary coded values into a white zero or black one for each print position of the matrix. Also shown is the incorporation of an alternating bit in the logic to allow estimation of an eight-level quantization by a four bit matrix through the incorporation of an extra black dot in every other matrix. In U.S. Pat. No. 3,604,846, David Behane et al, entitled "Method and System for Reconstruction of Half-Tone Images," the matrix patterns are placed in a table in storage and are accessed in accordance with a quantized video density value.
The difficulty with the dot bunching arrangement of Sweet et al is that it generates pronounced moire patterns in any larger area of the same or similar gray scale levels. Judicious choice of the matrices of Machol or of Behane et al may reduce but do not eliminate the Moire patterns. They, however, introduce the requirement for complex logic circuitry or for an extensive storage and accessing mechanism to transform the quantized gray level into the proper form.
Purely random dot generation for halftones has been known for some time and a specific arrangement is shown in IBM Technical Disclosure Bulletin, "Halftone Image Produced by Pseudo-Random Bit Generator," G. L. Smith, Vol. 17, No. 7, Dec. 1974, pages 1858-1859. A high frequency pseudo-random bit generator partially enables various AND circuits which are also partially enabled by the gray scale level inputs so that a binary print output signal is more probable with higher gray scale levels. Being purely random, the system may convert at times the wrong gray scale. The circuitry requires the gray scale input to cover a large scanned area and the system lacks high resolution capability for black and white printing.